Yes....and no.
Magnepans are 'voiced' under certain listening conditions with certain upstream equipment.
You CAN use the resistor to attenuate HF output. The DIY crowd is also, for some reason, a big fan of Low DCR inductors in the bass, which raises bass output in relation to HF output, and therefore has the same effect.
I hate to bring up ohms law, but simply put, a resistor in series with another, in this case say....1 ohm in series with 4 ohms, will take away about 20% of the power which formerly went to the 4ohm load only.
Just my opin, but have you exhausted setup potential yet? A couple 'fake ficus' behind the panels or perhaps a staggered bookshelf or even a small tapestry or maybe flipping them around would take off the HF edge.
The flipping worked wonders in my room and I'm now about to spend some money on some home built/designed sound panels. I may post a photo or design.
If anyone has Google Sketchup, I'll send a simple drawing of the panel and how to join the wood.....